Glyphosate-resistant Palmer amaranth weed present in Kansas?

Kansas State University scientists say there is evidence to show that some Palmer amaranth weeds in Kansas are resistant to the widely-used herbicide, glyphosate.

“Glyphosate-resistant weeds continue to be an increasing problem in Kansas,” said K-State agronomist Dallas Peterson. “Glyphosate-resistant marestail, common waterhemp, common ragweed, giant ragweed, and kochia have been previously confirmed in Kansas and have become very problematic in certain areas. Palmer amaranth is a serious weed problem in Kansas but until now, glyphosate-resistant Palmer amaranth had not been confirmed in the state.”

Glyphosate-resistant Palmer amaranth is a serious problem in the Southeast U.S., said Peterson, who is a weed specialist with K-State Research and Extension, adding that it has dramatically impacted weed control programs and even cropping systems.

Hot, dry weather over the past two years has made it difficult to assess herbicide performance and resistance problems, but poor control of Palmer amaranth with glyphosate has raised questions about whether glyphosate-resistant Palmer amaranth populations are now showing up in Kansas.

To determine if that is the case, K-State graduate student Josh Putman, working under Peterson’s direction, collected waterhemp and Palmer amaranth seed in the fall of 2011 from various soybean and cotton fields in eastern and south central Kansas. Waterhemp and Palmer amaranth plants from these different populations were grown in a K-State greenhouse and treated with glyphosate at one, two, and four times the typical field use rate of 0.75 pounds acid equivalence per acre to evaluate for resistance. The response of the different populations was compared to susceptible populations of both species collected from the Ashland Bottoms experiment field south of Manhattan. The susceptible check populations of both species were completely controlled by all rates of glyphosate.

“As expected, a number of the waterhemp populations from across eastern Kansas survived glyphosate treatment up to the four-times rate, and appeared to be resistant,” Peterson said. “Two populations of Palmer amaranth from Cowley County in south central Kansas also had a high percentage of plants that survived the one-time and two-times rates of glyphosate, and had some plants that survived the four-times rate. These Palmer amaranth populations did not appear to be as resistant to glyphosate as some of the Palmer amaranth from the Southeast U.S., but weren’t being controlled by typical field rates in the greenhouse or with multiple applications of glyphosate in the field.

So, it appears that glyphosate-resistant Palmer amaranth may now be present in Kansas, he said.

Palmer amaranth seed was collected from additional fields this past fall and is now being evaluated in a K-State greenhouse.

“Preliminary observations suggest that we are seeing similar survival after glyphosate treatments from Palmer amaranth collected from fields south of Great Bend, along with additional populations collected south of Wichita,” Peterson said. “Research on inheritance and the mechanism of resistance will need to be conducted to further characterize and confirm glyphosate resistance in these populations.”

Confirming herbicide resistance is a long and detailed process, he added.

“Regardless of whether glyphosate-resistant Palmer amaranth is now present in any given area in Kansas, there is a good chance it will develop at some point based upon what has happened in the Southeast U.S., especially if growers rely heavily on glyphosate for weed control,” Peterson said.

“Palmer amaranth is an extremely competitive weed, and the development of glyphosate resistance means it will require an effective integrated weed management program to achieve acceptable control,” he said. “Continuing to rely only on glyphosate for weed control will only speed up the development of glyphosate-resistant weeds and diminish its effectiveness.”

He said that the use of residual herbicides with different modes of action throughout the cropping system will help to manage existing glyphosate-resistant weeds and slow the development of new glyphosate-resistant weed populations.